In R12, the payments process has undergone quite a bit of change. Earlier in R11i, one could only process payments for one OU, because the internal (disbursing) bank was associated to an OU. However, in R12, internal banks are set for an LE which could be in turn associated to one or more OU's. Hence, its possible that while doing a payment run, you can process all invoices from your internal bank across invoices raised under different OU's.